For 2013 return, you should log in CRA " my account" and check if there is an amended T4 over there? If there is , that means an amended T4 has been already issued, but for some reasons, you did not get it. In this case, your return should be filed base on the amended T4.
If the T4 posted in the CRA same as the one you have, which you do not agree with, and you choose to file your return base on the actual info. you agree with, you might need to prepare something to show to the CRA.
To do these, you can copy all the pay stubs you have, your bank statements ( marked every net pay from this employee), point out which pay period is missing, along with a copy of the ROE form, the CRA will figure out the correct amounts should be reported in box14, 16, 18, 22. Apparently, the employer will get auditted by the CRA.
You mentioned that you have the records for all pay periods. Did you have the total amounts for CPP, EI, income tax deducions? If you do, you can file your return right away. If you don't, you might need to wait the responses from the CRA, and you might miss the deadline on May 5th.
For my understanding, if the employer issued an incorrect T4, there must be a discrepancy exits, and it will be discovered in the future day either by the employer or the CRA.